Hi Dave,
Checked on two new original Ex-Holder unused radiators I have – and you are correct, the measurement is 7.5 inches, yet the rad I measured first which came with the bike which is off for re-plating is 7.25 inches!!!
I think that by 1948 the limit guages must have been worn out.
P.S.
Spoke to Northern Radiators some while back – and I recall they said the rads as supplied to Scotts at Shipley were three pounds seven and sixpence each. Happy Days!!
